Don't call wxDocument::OnCloseDocument() twice when closing

When closing the document using wxDocManager::CloseDocument(), which is
what happens when using the standard wxID_CLOSE menu command handler,
OnCloseDocument() was called twice: first when wxDocument::Close() was
called directly from CloseDocument() itself and once again later when it
was implicitly called from DeleteAllViews().

This was unexpected and also resulted in calling DeleteContents() twice,
which is even more so for a function supposed to perform cleanup.

Fix this by not calling Close() itself but instead a new CanClose()
function from CloseDocument() to check whether we can close and then
relying on DeleteAllViews() to actually close the document.

docs/changes.txt

@@ -11,6 +11,12 @@ INCOMPATIBLE CHANGES SINCE 3.2.x:
 Changes in behaviour not resulting in compilation errors
 --------------------------------------------------------
 
+- wxDocument::OnCloseDocument() was called twice in previous versions when
+  closing the document from the menu. Now it is only called once and after
+  destroying all the existing document views. If you overrode this function,
+  please check that you don't rely on any views existing when it's called.
+
+
 Changes in behaviour which may result in build errors
 -----------------------------------------------------